The present invention relates to the art of earth boring and more particularly to an earth boring cutter with a thread-on replaceable cutting element.
Conventionally, rotary drilling apparatus and particularly rotary drilling apparatus for boring large diameter holes and tunnels includes a multiplicity of roller cutters. A number of the roller cutters together with their bearings and the saddles in which the cutters and bearings are mounted are positioned on a rotary head or a bit body and used to form holes in the formation being bored. The roller cutters may be conveniently mounted on the rotary cutting head of a tunneling machine or on the body of a raise bit. The cutters may be disc type cutters or full face cutters.
A disc type roller cutter is effective in very soft to medium formations. The disc is usually a heat treated alloy steel cutter with an included angle between about 60.degree. and 90.degree.. The disc cutters are usually indexed two or three inches apart. In effect, the disc cutters plow concentric circles around the face of the formation being bored. The cutting discs are indexed so that the formation between discs will break out completely under a given load and R. P. M. This is a very efficient way to cut formations because the cuttings come off the face in relatively large pieces. Disc cutters are not economical in harder formations because the discs dull out quickly in the harder abrasive formations. This is especially detrimental in shaft drilling or raise drilling operations where trip time is costly. It is not practical to make the discs completely of carbide and the brazing on of continuous sintered carbide tips or wedges is also of questionable feasibility from an economical and operational standpoint. Since the bearing life of the roller cutters long outlasts the life of the cutting structure, the cutting structure should be replaced periodically thereby extending the useful lifetime of the cutter. Replacement should be easily possible in the field.